Metal tubes are widely used in important fields such as national defense and military, aerospace, petrochemical, and marine engineering equipment including rocket engine propellant refueling systems, oil and gas transportation pipelines, and plunger sets of deep-sea sea pumps, which play an important role in guaranteeing national security and industrial development. However, in harsh environments, the inner walls of tube fittings often face serious damage such as corrosion, scratching and abrasion, which not only leads to shortened service life, but also brings huge economic losses. The design and preparation of multifunctional protective coatings with smooth and dense surface, high hardness, and strong film-based bonding inside the tube has become an urgent need. A stainless steel tube with a length of 300 mm, an outer diameter of 105 mm, and an inner diameter of 100 mm was used as the substrate, and polished and cleaned in turn. Seven locations 5, ?5, 10, ?10, 15 and ?15 cm from the middle of the tube were selected to characterize the structural and mechanical properties of the 304 stainless steel tube coated with Si/O-DLC film inside the tube. Different working pressures (15 mTorr, 20 mTorr and 25 mTorr) were changed to explore the effects on the plasma glow discharge in the tube and the structure and properties of the thin film. Firstly, a high-speed camera (Photron fastcam MiNi 100, Photron Co, Japan) was used to observe Plasma discharge optical phenomena. Secondly, scanning electron microscope and atomic force microscope were used to observe the surface morphology and roughness of the thin film and measure the thickness of the thin film. Then, the microstructure and mechanical properties of the film in the tube were analyzed by Raman spectroscopy, nanoindenter, and scratch testing system. With the increase of the working pressure, the bright spot area and light intensity at the center of the tube diameter firstly increased and then tended to shrink and fade. The average thickness of Si/O-DLC films in the tube increased from 1.42 μm to 2.06 μm with the increase of working pressure, and the uniformity of film thickness along the axial direction of the tube increased significantly from 24% to 65%; and the average hardness of Si/O-DLC films in the tube at different working pressure increased firstly and then decreased. The average hardness of inside Si/O-DLC films of the tube tended to increase and then decrease along the axial direction of the tube at different working pressure, with an overall average hardness of (14±1) GPa. The inside Si/O-DLC films of the tube obtained a high average film-base bonding at working pressure up to 25 mTorr. The distribution of the thickness and roughness of the Si/O-DLC film along the axial direction of the tube has the best uniformity at 25 mTorr. As the working pressure rises, the average thickness of the film on the inner wall of the tube along the tube axis increases, and the uniformity is improved. The overall roughness value of the surface morphology of the film in the tube is small and varies in a small range. In general, the roughness distribution of the film in the tube under different working pressure is relatively uniform, and the film and the substrate are closely bonded without microscopic defects.